Editing Sea level (section)

{{Short description|Geographical reference point from which various heights are measured}}
{{Other uses}}
{{Use dmy dates|date=September 2017}}
[[File:Israel Sea Level BW 1.JPG|thumb|upright=1.35|This marker indicating sea level is situated between [[Jerusalem]] and the [[Dead Sea]].]]

'''Mean sea level''' ('''MSL''', often shortened to '''sea level''') is an [[mean|average]] surface level of one or more among [[Earth]]'s coastal [[Body of water|bodies of water]] from which heights such as [[elevation]] may be measured. The global MSL is a type of [[vertical datum]]{{spaced ndash}}a standardised [[geodetic datum]]{{spaced ndash}}that is used, for example, as a [[chart datum]] in [[cartography]] and [[Navigation|marine navigation]], or, in aviation, as the [[standard sea level]] at which [[atmospheric pressure]] is measured to [[Calibration|calibrate]] altitude and, consequently, aircraft [[flight level]]s. A common and relatively straightforward mean sea-level standard is instead a long-term average of [[tide gauge]] readings at a particular reference location.<ref>[https://psmsl.org/train_and_info/faqs/#1 ''What is "Mean Sea Level"?''] Liverpool, UK: [[National Oceanography Centre]]. Retrieved 29 January 2024.</ref>

The term ''above sea level'' generally refers to the [[height above mean sea level]] (AMSL). The term '''APSL''' means above present sea level, comparing sea levels in the past with the level today.

Earth's radius at sea level is 6,378.137&nbsp;km (3,963.191&nbsp;mi) at the equator. It is 6,356.752&nbsp;km (3,949.903&nbsp;mi) at the poles and 6,371.001&nbsp;km (3,958.756&nbsp;mi) on average.<ref>{{cite web |title=Earth Radius by Latitude Calculator |url=https://rechneronline.de/earth-radius/ |access-date=22 August 2021 |archive-date=15 August 2021 |archive-url=https://web.archive.org/web/20210815042828/https://rechneronline.de/earth-radius/ |url-status=live}}</ref> This [[Reference ellipsoid|flattened spheroid]], combined with local [[gravity anomalies]], defines the [[geoid]] of the Earth, which approximates the local mean sea level for locations in the open ocean. The geoid includes a significant [[Indian Ocean Geoid Low|depression in the Indian Ocean]], whose surface dips as much as {{cvt|106|m|0}} below the global mean sea level (excluding minor effects such as tides and currents).<ref name="Geoid">{{cite journal |last1=Sreejith |first1=K.M. |last2=Rajesh |first2=S. |last3=Majumdar |first3=T.J. |last4=Rao |first4=G. Srinivasa |last5=Radhakrishna |first5=M. |last6=Krishna |first6=K.S. |last7=Rajawat |first7=A.S. |title=High-resolution residual geoid and gravity anomaly data of the northern Indian Ocean – An input to geological understanding |journal=Journal of Asian Earth Sciences |date=January 2013 |volume=62 |pages=616–626 |doi=10.1016/j.jseaes.2012.11.010|bibcode=2013JAESc..62..616S }}</ref>